The present invention relates to a method and apparatus for controlling a PWM inverter used in a variable speed drive of an induction motor.
To operate at high efficiency over a wide range of speed, an adjustable-speed ac motor is provided with an adjustable-voltage and adjustable frequency power source. The voltage and frequency are not totally independent and are generally varied in such a way as to maintain a constant ratio of voltage to frequency. That is, higher ac voltage and higher frequency result in higher motor speed. It is typical to provide power to the motor with an adjustable-voltage, adjustable-frequency inverter. However, since the inverter operates from a relatively fixed source of dc voltage, control of the inverter output voltage must be provided in or associated with the inverter. The preferred technique is to pulse-width modulate the output voltage.
Various forms of pulse-width-modulation (PWM) have been proposed, with a form of sine-wave shaping being a preferred approach. Rapid advancements in microprocessor-based control systems and stored-program memory devices have reduced their costs and complexity to the point where they are used to generate the pulse-width-modulation switching times.
To produce a pulse width modulated (PWM) pulse used to control a PWM inverter, two systems are generally used, one is an asynchronous and the other is a synchronous system in which the modulation wave is in sychronism with the carrier. In the asynchronous system, the PWm pulse is produced while the carrier is not synchronized with a particular phase of an output voltage of the inverter. On the other hand, in the synchronous system, the PWM pulse is produced while the carrier is always synchronized with a particular phase of the output voltage of the inverter independently from an output frequency of the inverter. In order to produce the PWM pulse in this manner, a sine wave of the modulation wave is compared with a ramp wave of the carrier. In the pulse width control, harmonics are included in the output current of the inverter. In order to supress the harmonics, a frequency of f.sub.c of the carrier is selected higher than a frequency f of the modulation wave (which is equal to a frequency of the inverter output voltage). In other words, a frequency ration f.sub.c /f is selected to be large. However, when the frequency f.sub.c /f is constant, a loss of a switching element of the inverter increases in a high frequency band. Accordingly, the frequency ratio f.sub.c /f is selected to be large in a low frequency band and gradually decreased as the inverter output frequency increases. This is disclosed in Japanese patent application, JP-A-60-174088.
The problems that have been encountered with these prior systems are that in the synchronous system, each time the frequency ratio f.sub.c /f changes, an overcurrent is generated and a torque is varied. Further, since harmonics of the output current change, magnetic noise of the motor changes and it presents an uncomfortable feeling.
On the other hand, in the asynchronous system, the carrier frequency f.sub.c may be set constant because the modulation wave and the carrier are asynchronous. As a result, the frequency ratio f.sub.c /f continuously changes as the inverter output frequency f changes, and no trouble such as torque variation occurs. However, as an amplitude ratio K.sub.H of the modulation wave and the carrier is gradually increased to increase the inverter output voltage, the output current oscillates. The oscillation phenomenon has been confirmed by an experiment by the inventors.
It is an object of the present to provide method and apparatus for controlling a PWM inverter which do not cause a torque variation and a change of noise tone of the motor when the pulse is switched, and can stably drive the motor irrespective of the magnitude of the output voltage.